The invention concerns a drive unit, in particular for a closing unit, an injection unit, or an ejector of an injection molding machine.
Recently there is a tendency to provide injection molding machines with electric and hydraulic drive mechanisms, wherein high-velocity actuation movements are carried out by the electric drive mechanism at comparatively low forces, while the hydraulic drive mechanism is particularly advantageous when high axial forces must be applied at comparatively minor actuation movements.
Such requirements occur, for instance, in a closing unit or an injection unit or in the actuation of ejectors of a plastics injection molding machine. For example in order to inject plastics into a tool cavity, a preplastifying spindle is moved at a relatively high axial velocity in a direction towards the mold until the mold is completely filled with plastic material. During the subsequent dwell pressure phase, in which workpiece shrinkage is compensated by supplying molding material, a high force must be applied via the injection unit in the absence of a substantial axial advance of the preplastifying spindle.
The requirements are similar in a closing unit for closing the mold of an injection molding machine, wherein the movable platen is initially moved rapidly for closing the mold through the intermediary of the electric drive mechanism, and then only a short residual stroke must be passed through for completely closing the molding tool and keeping it closed with the required closing force.
From DE 101 21 024 A1 to the present applicant a drive unit of an injection molding machine is known, wherein the comparatively high axial forces while keeping the injection molding tool closed or during the dwell pressure phase upon injection are applied through the intermediary of a hydraulic power converter. This hydraulic power converter includes a piston unit having a comparatively small effective surface that is actuated through a spindle arrangement. The smaller piston unit delimits a pressure chamber jointly with another piston unit having a larger effective surface and an axially displaceable cylinder. The cylinder may be locked for the application of high axial forces, so that due to the axial movement of the smaller piston unit the pressure chamber is reduced in size and, in accordance with the difference of surface area between the smaller piston unit and the larger piston unit, a high axial force is applied by the latter onto the unit to be actuated (closing unit/injection unit/ejector). Axial fixation of the cylinder of the power converter is effected, in the known solution, in that a cylinder wall is bulged in a radially outward direction by the rising pressure in the pressure chamber, so that the cylinder is held by frictional grip in a guide means of the cylinder and thus cannot perform an axial movement any more.